1. Field of the Invention
The present invention relates to a down-shift control system used in a vehicle having an automatic transmission.
This application is based on Patent Application No. Hei 9-231583 filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Conventionally, instead of a mechanical system in which an amount of force imposed on an accelerator pedal and a degree of opening of a throttle valve are changed in a mechanically interlocking situation, an electric control system for controlling a throttle valve is known in which the degree of opening of a throttle valve with respect to the amount of force imposed on an accelerator pedal is automatically adjusted based on a driving condition.
Japanese Patent Application, First Publication, No. Hei 5-139188 discloses the following shift-switching control based on such a throttle control system performing electric control (hereinafter, simply called the "throttle control system"). The disclosed technique is such that a down-shift operation is executed in case of need, according to a first judgment following a shift map which indicates each optimal shift position based on a relationship between the degree of opening of the throttle valve and the speed of the vehicle, and to a second judgment for performing a down-shift operation if the engine does not have fully enough torque, based on a difference between the maximum torque and target torque at the relevant shift position.
However, the driving situation of the vehicle is considerably influenced according to which of the two judgments (i.e., one based on the shift map and the other based on the amount of residual torque) is given priority for switching the shift position.
Practically, if the judgment based on the shift map is given priority, the downshift operation is never performed unless the operational line on the shift map crosses a boundary corresponding to a switching operation. Therefore, the down-shift switching operation cannot follow the manipulation of the accelerator pedal, and thus the driving capability is lowered. On the other hand, if the judgment based on the residual torque is given priority, a down-shift operation is performed even for an instantaneous insufficiency of the engine torque and according to this down-shift operation, surplus engine torque is generated and an up-shift operation is next performed. That is, the switching operation is repeatedly performed (i.e., a "shift- busy" phenomenon occurs).